The notion that mitochondrial encephalomyopathies are important causes of mental retardation in childhood that has been reinforced in the past four years by numerous studies, including those conducted as part of our Program Project. While our original application focused on disorders associated with point mutations of mtDNA, we will extend our interest to mitochondrial diseases due to mutations in the nuclear genome, specifically to the most common form of Leigh syndrome (LS), due to cytochrome c oxidase (COX) deficiency. We will also build on the experienced gained in the past 4 years to develop therapeutic approaches, at both the clinical and investigative levels. Project 1 (D.C. De Vivo, PI) will continue to characterize the natural history of MELAS and MERRF, correlating cognitive, and behavioral deficits in proband and oligosymptomatic maternal relatives with cerebral energy metabolism assessed by functional MRI and MRSI. Taking advantage of the large cohort of genetically homogeneous patients enrolled in the past four years, it will institute a controlled clinical trial with dichloroacetate (DCA), following brain lactate and neuropsychological features. Project 2 (S. DiMauro, PI) will add a molecular dimension to Project 1, by following mutational loads and drifts in heteroplasmy in various tissues from MELAS and MERRF patients, and by verifying the effects of DCA in vitro on cell lines harboring the MELAS mutation. It will extend promising preliminary data on disorders due to mutations in newly-identified mtDNA protein- coding genes. It will screen tissues from about 50 patients with COX- deficient LS for mutations in nuclear "COX-assembly" genes. It will screen tissues from about 50 patients with COX-deficient LS for mutations in nuclear "COX-disorders due to mutations in mtDNA protein-coding genes, and will attempt to devise a pharmacological approach to these disorders directed towards "downshifting" the percentage of mutant mtDNAs. Project 4 (M.M. Davidson, PI) will concentrate on genetic and functional complementation between the MELAS and MERRF mutations in syncytial myotubes and myofibers in culture, and explore the roles of mitochondrial fusion, mitochondrial fission, for the MELAS and MERRF mutations, using immunohistochemical techniques, focusing on the blood-brain proteins in brains from normal children and from children with COX-deficient LS. The Core Unit (S. DiMauro, Director) will provide direction, administration external consultation, and shared equipment/technical service to the project as a whole. Dr. Moraes, Rapin and Shoubridge will constitute the External Advisory Committee.